System and method for controlling scale build-up in a wfgd

ABSTRACT

The present invention relates generally to the cleaning of a flue gas and, in one embodiment, to a device, system and method that mitigates and/or prevents slurry deposition at the flue inlet to a wet flue gas desulfurization (WFGD) unit in order to keep the inlet dry and minimize deposition (e.g., deposition scale) at the flue inlet to the WFGD tower. In one embodiment, a wet flue gas desulfurization (WFGD) unit, system and/or method according to the present invention comprises, among other features, a scale prevention system comprising a plenum air device and/or forced-air box located proximate the inlet transition zone. In another embodiment, a wet flue gas desulfurization (WFGD) unit, system and/or method according to the present invention comprises, among other features, a scale prevention system comprising at least one chill plate located proximate the inlet transition zone.

RELATED APPLICATION DATA

This patent application claims priority to U.S. Provisional PatentApplication No. 61/717,133 filed Oct. 23, 2012 and titled “System andMethod for Controlling Scale Build-Up in a WFGD.” The complete text ofthis application is hereby incorporated by reference as though fully setforth herein in its entirety.

FIELD AND BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the cleaning of a flue gasand, in one embodiment, to a device, system and method that mitigatesand/or prevents slurry deposition at the flue inlet to a wet flue gasdesulfurization (WFGD) unit in order to keep the inlet dry and minimizedeposition (e.g., deposition scale) at the flue inlet to the WFGD tower.In one embodiment, a wet flue gas desulfurization (WFGD) unit, systemand/or method according to the present invention comprises, among otherfeatures, a scale prevention system comprising a plenum air deviceand/or forced-air box located proximate the inlet transition zone. Inanother embodiment, a wet flue gas desulfurization (WFGD) unit, systemand/or method according to the present invention comprises, among otherfeatures, a scale prevention system comprising at least one chill platelocated proximate the inlet transition zone. In still anotherembodiment, a wet flue gas desulfurization (WFGD) unit, system and/ormethod according to the present invention comprises, among otherfeatures, a scale prevention system comprising a plenum air deviceand/or forced-air box and at least one chill plate, where both devicesare located proximate the inlet transition zone.

2. Description of the Related Art

FIG. 1 is an illustration of a typical wet flue gas desulfurization(WFGD) unit and the various parts and features contained therein. Asillustrated in FIG. 1, the SO₂-laden flue gas that enters via the inletshown in FIG. 1 in various WFGD towers comes into contact with theslurry utilized in the WFGD tower. Due to the hot flue gas entering thetower, the flue gas temperature and the gas flow path around the inletcan cause the slurry to deposit on the walls/roof of the inlet nozzle.The slurry can then “flash-dry” onto this surface thereby leaving one ormore deposits extremely hard scale. This scale has the potential tobreak off in-service during tower load swings and plant outages therebycausing one or more pieces of hard and abrasive material to fall intothe WFGD tank and make its way through the process streams.

While not wishing to be bound to any one theory, it is believed that theformation of the scale starts in the two lower corners of the WFGD inletwith the lowest gas flow, and thus, the highest possibility ofstagnation and highest potential of liquid to gas in the inlet. Onenon-limiting theory is that in the specific case of tile-lined WFGDtowers allow for more adhesion of the scale due to the porous nature ofthe grout and/or tile utilized therein. This allows for large pieces ofscale to form and shed at one time.

Given this, one potential solution to the above problem is disclosed inU.S. Pat. No. 5,403,523 where a deflection system is utilized tomaintain a cleaner WFGD inlet. Given this, an exemplary prior artsolution is illustrated in FIGS. 2 through 5.

Turning to Figures, FIG. 1 is an overall view of an exemplary wet fluegas desulfurization (WFGD) unit, while FIG. 2 is a cross-sectionalillustration of a portion of a known wet flue gas desulfurization (WFGD)system 10, wherein a wet scrubber module 12 having a cylindrical housing14 receives a flue gas 16 at a transition between an inlet flue 18 andthe housing. Inlet flue 18 typically approaches cylindrical housing 14at a downward angle ranging from 0° to 90° from the horizontal. Inletflue 18 is typically rectangular in cross-section, having a width W thatis approximately 2.5 times greater than its height H (i.e., an aspectratio of width to height W/H of approximately 2.5). At locations 20, 22near an intersection of upper and lower surfaces 24, 26, respectively ofinlet flue 18 and cylindrical housing 14, inlet flue 18 is mitered sothat it intersects and attaches to cylindrical housing 14 at a 90°angle; i.e., inlet flue 18 is substantially perpendicular to cylindricalhousing 14. This perpendicular orientation simplifies the transition,structural design and fabrication of the wet scrubber module 12. Priorto inlet flue 18, other portions (not shown) of the flue system upstreamof inlet flue 18 may include cross-section transitions, elbows, fans,and/or other hydraulic devices to supply the flue gas 16 to the inletflue 18.

As illustrated in FIGS. 3A and 3B, within the cylindrical housing 14, aninlet awning 28 is used to direct part of the hot flue gas 16, typicallyprovided to the wet scrubber module 12 at a temperature of approximately300° F., downwardly at a 45° angle with respect to horizontal. Liquidslurry 30 that is sprayed within the wet scrubber module 12 and/or whichdrains from packing, trays, or other surfaces within the wet scrubbermodule 12, flows downwardly onto an upper surface 32 of the inlet awning28. A weir plate 34 located above a tip 36 of the inlet awning 28provides a gap 38 through which liquid slurry 30 passes, and thusregulates and improves the uniformity of a liquid slurry curtain 40 thatis discharged down into the hot flue gas stream 16. As the hot flue gas16 enters the wet scrubber module 12, it is quenched and humidified bythe liquid slurry curtain 40, evaporating a fraction of the waterpresent in the liquid slurry curtain 40.

In the absence of an inlet awning 28, a thin liquid film of liquidslurry 30 flowing downwardly along the walls of the wet scrubber module12 contacts the hot flue gas 16. However, the amount of water present inthe thin liquid film of slurry 30 falling across the inlet opening tothe wet scrubber module 12 is not sufficient to completely quench andhumidify the hot flue gas 16. The inlet awning 28 collects all theliquid slurry 30 that falls on top of it and channels the liquid slurry30 to the gap 38 to form the thick liquid slurry curtain 40. This largeramount of water diverted by the inlet awning 28 and forming the liquidslurry curtain 40 exceeds what is needed for complete and instantaneousquenching and humidification of the hot flue gas 16. This complete andsudden quench and humidification reduces the wet/dry interface and thepossibility of solid deposition.

In the absence of an inlet awning 28, if the liquid slurry 30 and hotflue gas 16 combination contacts any surface of the inlet flue 18 orhousing 14, solid deposits will form as the liquid slurry 30 evaporates.Solids form at the wet/dry interface because the water present is notsufficient to continuously and fully humidify the incoming gas. Thesesolid deposits build up over time, which requires the wet scrubbermodule 12 to be shut down so that maintenance personnel can enter thewet scrubber module 12 and remove the deposits. Shut down of a wetscrubber module 12 requires that either spare wet scrubber modules 12 beavailable to clean the flue gas 16, that the boiler load is reduced sothat the amount of flue gas produced does not exceed the capacity of theremaining on-line wet scrubber modules 12, or that there is a dischargeof partially or untreated flue gas 16 directly into the atmosphere. Allof these alternatives are undesirable and not accepted by the industry.It is thus preferable to maintain the wet/dry interface of the liquidslurry 30 with the hot flue gas 16 away from these surfaces, and theinlet awning 28 accomplishes this result by creating the liquid slurrycurtain 40. The liquid slurry curtain 40 is maintained away from thesesurfaces because the inlet awning 28 extends downwardly and into thecylindrical housing 14 of wet scrubber module 12. The liquid slurrycurtain 40 also provides more water than required for humidification.

As shown in FIGS. 4 and 5, at the side ends 42 of the inlet awning 28,sidewalls 44 extend from the inlet awning 28 into the housing 14 anddownwardly to a point below the lower surface 26 of inlet flue 18. Thesesidewalls 44 prevent liquid slurry 30 from flowing off the side ends 42of the inlet awning 28 or along an inner surface 46 of the housing 14from entering inlet flue 18. These sidewalls 44 are necessary tomaintain an acceptable wet/dry interface away from these surfaces toavoid the deposition problems discussed earlier. In addition, the inletawning 28 is provided with stiffeners 48 which, in conjunction with thesidewalls 44, further distribute the slurry flow 30 evenly about thesesurfaces.

Hydraulic testing of the known inlet flue 18 and inlet awning 28 devicesdescribed above revealed a significant flue gas side total pressuredrop. High flue gas side pressure drops require increased fan pressurecapability, resulting in increased fan and motor capacity and increasedoperating costs for the life of the unit. This is very undesirablebecause even a 1.0 inch H₂O gas side pressure drop can be assessed atvalues which can reach one million dollars. Therefore, reducing the fluegas side pressure drop in the wet scrubber equipment is an effective wayto reduce costs. However, such reductions must still be achieved in amanner which prevents unwanted deposition of dried slurry material atthe transition interface and/or zone.

Given the above, a need exists in the art for a device, system and/ormethod by which to reduce the amount of slurry material that deposits inthe transition interface and/or zone where the flue gas inlet meets theWFGD tower.

SUMMARY OF THE INVENTION

The present invention relates generally to the cleaning of a flue gasand, in one embodiment, to a device, system and method that mitigatesand/or prevents slurry deposition at the flue inlet to a wet flue gasdesulfurization (WFGD) unit in order to keep the inlet dry and minimizedeposition (e.g., deposition scale) at the flue inlet to the WFGD tower.In one embodiment, a wet flue gas desulfurization (WFGD) unit, systemand/or method according to the present invention comprises, among otherfeatures, a scale prevention system comprising a plenum air deviceand/or forced-air box located proximate the inlet transition zone. Inanother embodiment, a wet flue gas desulfurization (WFGD) unit, systemand/or method according to the present invention comprises, among otherfeatures, a scale prevention system comprising at least one chill platelocated proximate the inlet transition zone. In still anotherembodiment, a wet flue gas desulfurization (WFGD) unit, system and/ormethod according to the present invention comprises, among otherfeatures, a scale prevention system comprising a plenum air deviceand/or forced-air box and at least one chill plate, where both devicesare located proximate the inlet transition zone.

Accordingly, one aspect of the present invention is drawn to a scaleprevention system for a wet flue gas desulfurization unit (WFGD), thesystem comprising: a flue structure, wherein the flue structure isconnected to the inlet of an wet flue gas desulfurization unit; at leasttwo air nozzles, each air nozzle having a respective air supply line,wherein the at least two air nozzles are positioned on the interior ofthe flue so as to provide air coverage across the horizontal width ofthe flue inlet to the wet flue gas desulfurization unit; and at leastone inlet awning designed to deflect a slurry from the wet flue gasdesulfurization unit, the at least one inlet awning being position abovethe flue inlet to the wet flue gas desulfurization unit.

In yet another aspect of the present invention, there is provided ascale prevention system for a wet flue gas desulfurization unit (WFGD),the system comprising: a flue structure, wherein the flue structure isconnected to the inlet of an wet flue gas desulfurization unit; at leasttwo air nozzles, each air nozzle having a respective air supply line,wherein at least one air nozzle is positioned on each opposite verticalinterior wall of the flue so as to provide air coverage across thehorizontal width of the flue inlet to the wet flue gas desulfurizationunit; and at least two lateral air nozzles, each lateral air nozzlehaving a respective air supply line, wherein each lateral air nozzle ispositioned internally of each of the at least two air nozzles and arepositioned at the top edge of the flue inlet to the wet flue gasdesulfurization unit so as to provide air coverage across the horizontalwidth of the flue inlet to the wet flue gas desulfurization unit.

In yet another aspect of the present invention, there is provided ascale prevention system for a wet flue gas desulfurization unit (WFGD),the system comprising: a flue structure, wherein the flue structure isconnected to the inlet of an wet flue gas desulfurization unit; and atleast two chill plates, each chill plate having a respective supplyline, wherein the at least two chill plates are positioned on theinterior of the flue so as to provide temperature control across thehorizontal width of the flue inlet to the wet flue gas desulfurizationunit.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific benefits attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich exemplary embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a typical wet flue gas desulfurization(WFGD) unit;

FIG. 2 is a cross-sectional illustration of the wet flue gasdesulfurization (WFGD) unit of FIG. 1;

FIG. 3A is a sectional side view of the transition between the inletflue and the wet scrubber module of FIG. 2;

FIG. 3B is a sectional view illustrating a portion of FIG. 3A;

FIG. 4 is sectional plan view taken in the direction of the 4-4 arrowsof FIG. 2;

FIG. 5 is a sectional side view illustrating a side wall for the inletawning of FIG. 2;

FIG. 6 is an illustration of one embodiment of a scale prevention systemfor a WFGD flue inlet;

FIG. 7 is a cross-sectional illustration of the scale prevention systemof FIG. 6;

FIG. 8 is an illustration of another embodiment of a scale preventionsystem for a WFGD flue inlet;

FIG. 9 is a cross-sectional illustration of the scale prevention systemof FIG. 8;

FIG. 10 is an illustration of still another embodiment of a scaleprevention system for a WFGD flue inlet;

FIG. 11 is a cross-sectional illustration of the scale prevention systemof FIG. 10;

FIG. 12 is a cross-sectional illustration of still another embodiment ofa scale prevention system for a WFGD flue inlet;

FIG. 13 is a cross-sectional illustration of still another embodiment ofa scale prevention system for a WFGD flue inlet; and

FIG. 14 is a cross-sectional illustration of still another embodiment ofa scale prevention system for a WFGD flue inlet.

DESCRIPTION OF THE INVENTION

The present invention relates generally to the cleaning of a flue gasand, in one embodiment, to a device, system and method that mitigatesand/or prevents slurry deposition at the flue inlet to a wet flue gasdesulfurization (WFGD) unit in order to keep the inlet dry and minimizedeposition (e.g., deposition scale) at the flue inlet to the WFGD tower.In one embodiment, a wet flue gas desulfurization (WFGD) unit, systemand/or method according to the present invention comprises, among otherfeatures, a scale prevention system comprising a plenum air deviceand/or forced-air box located proximate the inlet transition zone. Inanother embodiment, a wet flue gas desulfurization (WFGD) unit, systemand/or method according to the present invention comprises, among otherfeatures, a scale prevention system comprising at least one chill platelocated proximate the inlet transition zone. In still anotherembodiment, a wet flue gas desulfurization (WFGD) unit, system and/ormethod according to the present invention comprises, among otherfeatures, a scale prevention system comprising a plenum air deviceand/or forced-air box and at least one chill plate, where both devicesare located proximate the inlet transition zone.

As used herein, “inlet transition zone” is defined to mean the area inthe WFGD inlet and/or tower where the temperature gradient is changingfrom “hot” to “cold” and/or the flue gas is experiencing a change in theamount of water saturation in the flue gas due to contact of the fluegas with the slurry from the WFGD. As would be apparent to those ofskill in the art, the “inlet transition zone” is not located in onespecific area. Rather, the “inlet transition zone” varies from one WFGDunit to another, or can even vary within the same WFGD depending uponoperating conditions.

Regarding the air utilized in the various scale mitigation and/orprevention systems of the present invention, the air supplied to thevarious air lines and/or piping of any one or more embodiments of thepresent invention can be supplied from temperature controlled air fromany suitable source (e.g., from any suitable air pump, compressor,etc.), or can be supplied from any surplus oxidation air that might beavailable from the oxidation air being supplied to the WFGD. In anotherembodiment, the air supplied to the various air lines and/or piping ofany one or more embodiments of the present invention can be suppliedfrom any combination of temperature controlled air from any suitablesource (e.g., from any suitable air pump, compressor, etc.) and anysurplus oxidation air that might be available from the oxidation airbeing supplied to the WFGD.

Furthermore, the temperature of the air supplied to the various airlines and/or piping of any one or more embodiments of the presentinvention should be within a range of about −25° F. to about +25° F. ofthe WFGD slurry temperature. In another embodiment, the temperature ofthe air supplied to the various air lines and/or piping of any one ormore embodiments of the present invention should be within a range ofabout −20° F. to about +20° F., or about −15° F. to about +15° F., orabout −10° F. to about +10° F., or about −5° F. to about +5° F., or evenabout −2.5° F. to about +2.5° F. of the WFGD slurry temperature. Here,as well as elsewhere in the specification and claims, individual rangevalues and/or limits can be combined to form additional non-disclosedranges.

Turning to the Figures, FIG. 6 illustrates one embodiment of an inletscale mitigation and/or prevention system (hereinafter solely referredto as a “scale prevention system” for the sake of brevity). In theembodiment of FIG. 6, a scale prevention system 100 is disclosed. Asillustrated in FIG. 6, flue 102 is shown as entering the tower portion104 of a WFGD unit. It should be noted that although the tower portion104 of the WFGD is shown as having a circular cross-sectional shape, thepresent invention is not limited thereto. Accordingly, any geometricshape can be utilized for the tower portion 104 of a WFGD unit. Turningto the remaining portions of scale prevention system 100, flue 102 isformed so as to have at least one expansion joint 106 that traverses thewidth of flue 102. Additionally, scale prevention system 100 alsoincludes left and right air lines and/or piping 108 that supply air toleft and right side clean-out nozzles 110. Side clean-out nozzles 110direct clean-out air supplied via each clean-out nozzle's respective airline and/or piping 108 in the same general direction as the flue gasflow direction (denoted by arrow 112). In some embodiments, scaleprevention system 100 can further include an inlet awning and/orsidewalls like those disclosed and discussed in FIG. 5 so as to provideadditional coverage and/or protection to the top and/or side portions offlue 102 so as to prevent WFGD slurry “raining” down from above and thenentering flue 102. Accordingly, the combination of left and right airlines and/or piping 108, left and right side clean-out nozzles 110 and,if so present, inlet awning and/or sidewalls (see FIG. 5) act togetherto prevent WFGD slurry from entering into flue 102 and causing scale tobuild-up on the WFGD-end of flue 102.

FIG. 7 is a cross-sectional illustration of the open end of flue 102 asit enters the tower portion 104 of the WFGD illustrating the orientationof left and right clean-out nozzles 110. In another embodiment, if sodesired an inlet awning and/or sidewalls according to the embodiment ofFIG. 5 can be placed over the top and side edges of flue 102 (above theupper edges of left and right clean-out nozzles 110) to provide furtherprotection against the WFGD slurry “raining” down from above and thenentering flue 102. In still another embodiment, left and right clean-outnozzles 110 can be any desired height so long as left and rightclean-out nozzles start at the bottom edge of flue 102 and proceedsubstantially vertically up toward the top edge of flue 102.

Turning to FIG. 8, FIG. 8 illustrates another embodiment of a scaleprevention system 200. As illustrated in FIG. 8, flue 202 is shown asentering the tower portion 204 of a WFGD unit. It should be noted thatalthough the tower portion 204 of the WFGD is shown as having a circularcross-sectional shape, the present invention is not limited thereto.Accordingly, any geometric shape can be utilized for the tower portion204 of a WFGD unit. Turning to the remaining portions of scaleprevention system 200, flue 202 is formed so as to have at least oneexpansion joint 206 that traverses the width of flue 202. In anotherembodiment, the expansion joint can have one or more expansion jointdrip zones and/or drains 232 represented by the left and rightrectangular shapes. In one embodiment, expansion joint drip zones and/ordrains 232 are located on the underside of the expansion joint sectionof flue 202. Additionally, scale prevention system 200 also includesleft and right air lines and/or piping 234 that supply air to left andright side clean-out nozzles 236 as well as lateral clean-out nozzles238. Lateral clean out nozzles 238 are located toward the center line offlue 202 and are separated by baffle plate 240.

In one embodiment, lateral clean-out nozzles 238 and baffle plate 240are located along the bottom edge of flue 202 as it enters into tower204. In another embodiment, lateral clean-out nozzles 238 and baffleplate 240 are located along the top edge of flue 202 as it enters intotower 204. For the purposes of the present invention, the bottom edge offlue 202 is located closest to the bottom of tower 204. Baffle plate 240is placed between lateral clean-out nozzles 238 in order to createmultiple air zones due to the presence of at least two lateral clean-outnozzles. In one embodiment, the existence of baffle plate 240 permitseach of the left and right lateral clean-out nozzles 238 to be operatedindependently of one another thereby permitting customized control ofthe air provided to the left and right portions of flue 202 as it enterstower 204. In another embodiment, left and right clean-out nozzles 238can be operated in tandem rather than independently.

Given the above, the combination of side clean-out nozzles 236 andlateral clean-out nozzles 238 direct clean-out air supplied via eachclean-out nozzle's respective air line and/or piping 234 in the samegeneral direction as the flue gas flow direction (denoted by arrow 212).In another embodiment, although not illustrated, scale prevention system200 can further include an inlet awning and/or sidewalls thatcover/surround the top and/or side portions of flue 202 so as to preventWFGD slurry “raining” down from above and then entering flue 202.Accordingly, the combination of left and right air lines and/or piping234, left and right side clean-out nozzles 236, left and right lateralclean-out nozzles 238, and baffle plate 240 act together to prevent WFGDslurry from entering into flue 202 and causing scale to build-up on theWFGD-end of flue 202.

FIG. 9 is a cross-sectional illustration of the open end of flue 202 asit enters the tower portion 204 of the WFGD illustrating the orientationof left and right clean-out nozzles 236, left and right lateralclean-out nozzles 238, and baffle plate 240. In another embodiment, ifso desired an inlet awning and/or sidewalls can be placed over/aroundthe top and/or side edges of flue 202 (above left and right lateralclean-out nozzles 238 and baffle plate 240) to provide furtherprotection against the WFGD slurry “raining” down from above and thenentering flue 202.

Turning to FIG. 10, FIG. 10 illustrates another embodiment of a scaleprevention system 300. As illustrated in FIG. 10, flue 302 is shown asentering the tower portion 304 of a WFGD unit. It should be noted thatalthough the tower portion 304 of the WFGD is shown as having a circularcross-sectional shape, the present invention is not limited thereto.Accordingly, any geometric shape can be utilized for the tower portion304 of a WFGD unit. Turning to the remaining portions of scaleprevention system 300, flue 302 is formed so as to have at least oneexpansion joint 306 that traverses the width of flue 302. In anotherembodiment, the expansion joint can have one or more expansion jointdrains (not shown) located on the underside of the expansion jointsection of flue 302. Additionally, scale prevention system 300 alsoincludes left and right lines and/or piping 320 that supply temperaturecontrolled air and/or temperature controlled liquid to left and rightchill plates 322. The combination of left and right lines and/or piping320 and left and right chill plates 322 seek to reduce the temperatureat the inlet of WFGD tower 304 so as to mitigate and/or prevent scalebuild-up due to a large temperature change between the flue gas enteringthe WFGD and the WFGD's slurry that is “raining” down from above. Itshould be noted that in another embodiment of the present invention theembodiment of FIG. 10 can be combined with either of the embodiments ofFIG. 6 or FIG. 8.

In still another embodiment, although not illustrated, scale preventionsystem 300 can further include an inlet awning and/or sidewalls thatcover/surround the top and/or side portions of flue 302 so as to preventWFGD slurry “raining” down from above and then entering flue 302.

FIG. 11 is a cross-sectional illustration of the open end of flue 302 asit enters the tower portion 304 of the WFGD illustrating the orientationof left and right chill plates 322. In another embodiment, if so desiredan inlet awning can be placed over the top edge of flue 302 to providefurther protection against the WFGD slurry “raining” down from above andthen entering flue 302.

Turning to FIGS. 12 through 14, FIGS. 12 through 14 are cross-sectionalillustrations of various alternative embodiments of the presentinvention. In the embodiment of FIG. 12, a flue 402 and partial WFGDtower 404 are illustrated. As shown in the embodiment of FIG. 12, thescale prevention system disclosed therein contains both left and rightside clean-out nozzles 236 and left and right chill plates 322. In theembodiment of FIG. 13, the scale prevention system disclosed thereincontains left and right clean-out nozzles 236, left and right lateralclean-out nozzles 238, baffle plate 240, and left and right chill plates322 located as shown at the end of flue 502 as it enters tower 504. Inthe embodiment of FIG. 14, the scale prevention system disclosed thereincontains left and right clean-out nozzles 236, left and right lateralclean-out nozzles 238, baffle plate 240, and left and right chill plates322, with the left and right clean-out nozzles 236 being locatedvertically above the respective left and right chill plates 322 locatedas shown at the end of flue 602 as it enters tower 604. In still anotherembodiment, although not illustrated, any of the embodiments of FIGS. 12through 14 can further include an inlet awning and/or sidewalls thatcover/surround the top and/or side portions of flue 402/502/602 so as toprevent WFGD slurry “raining” down from above and then entering flue402/502/602.

It should be noted that the size, height and/or width of any of thenozzles, chill plates and/or baffle plates of the present invention canbe varied as needed and/or desired. Accordingly, the present inventionis not limited to just one geometric shape, layout, and or designorientation. Furthermore, the various nozzles, chill plates, and/orbaffle plates can be operated in combination, various sub-combinations,or even each item independently.

While not wishing to be bound to any one advantage, or set ofadvantages, a scale prevention system according to the present inventionprevents scale from forming and then falling into the WFGD tank.Alternatively, if scale forms the scale that forms is smaller in sizeand is able to broken-up into smaller pieces by the agitators and pump(absorber recycle pumps and/or bleed pumps). Given this, in the bleedpump stream, the scale pieces are able to be pumped into the primaryhydroclones. The one or more systems of the present invention preventlarger scale pieces from forming and thereby prevents such scale piecesfrom entering into the one or more hydroclones were such pieces are notable to be removed in the underflow stream. This reduces plugging of theone or more hydroclones and thus reduces the frequency of maintenanceintervals.

The present invention is also advantageous over those systems thatutilize some type of water-based system to prevent scale formation atthe tower end of a flue entering a WFGD. Such water-based systems aresubject to a number of drawbacks including, but not limited to,undesirable variations in the WFGD tower water/aqueous balance,corrosion at the tower end of the flue entering the WFGD; and/or watersupply problems that might actually end up causing an increase in scaleformation.

Another potential advantage attributable to the present invention is thereduction in scale size and frequency reduces the amount and size ofscale pieces that are forced through the Absorber Recycle (AR) pumps.This has the potential of reducing the amount of abrasive particles inthe pump stream. Furthermore, scale pieces are typically pushed into theAR headers and into the slurry nozzles in the upper portion of thetowers. These nozzles have small openings and if enough scale piecesbecome wedged together, there is a high likelihood of plugging ARnozzles and/or headers. Such plugging leads to a drop in SO₂ removal aswell as the risk of lower limestone utilization by the system.Accordingly, the one or more embodiments of the present invention canreduce the severity and/or occurrence of this phenomena therebyresulting in better SO₂ removal rates as well as less damage to the oneor more AR nozzles.

Given the above, the embodiments of the present invention utilize twodifferent principles to reduce the amount of scale that forms at theflue inlet of a WFGD. The embodiments of FIGS. 6 through 8 utilizepressurized and/or forced air via various nozzles and/or a plenumstructure to keep the inlet area clean by two means: (i) by higherpressure air forcing any slurry and/or eddy current gas flow out of theinlet and into the tower; and (ii) by supplying air that serves tocreate an insulating zone between the hot flue gas and cooler slurry. Inone embodiment, the air supplied to the flue inlet can be humidifieddown to an acceptable temperature. This colder plenum should be kept ata temperature equal to or less than the operating temperature of theWFGD fluid in order to prevent the slurry from flash-drying to theplenum and forming scale.

In still another embodiment, the air source for the various embodimentsof the present invention is provided from the WFGD's oxidation air flowinto the tower, then an added benefit is realized in that WFGD sites aredesigned and are operating with (due to tower loading turn-down) excessoxidation air. This excess oxidation air leads to adverse chemistryeffects in some plants including but not limited to: (a) strong oxidizerformation with downstream impacts; (b) tower corrosion from manganesedeposition forming a galvanic bridge to drive fluoride inducedunder-deposit corrosion; (c) adverse selenium speciation; and (d)mercury reemission. Accordingly, the use of such excess oxidation aircan help to reduce, mitigate and/or eliminate one or more of the abovenoted negative effects.

The embodiments of FIGS. 9 and 10 that utilize left and right chillplates, with or without the use of pressurized/oxidation air, candecrease the formation of the scale at the flue inlet to the WFGD. Thechill plates help to maintain a cooler inlet surface which helps toprevent the thermal shock the slurry experiences at the flue inlet. Thisthermal shock can lead to the “flash drying” that is linked to scaleformation at the flue inlet. The chill plates also permit the useflowing liquid/or gas at a predetermined temperature so as to keep thecontact surface of the inlet at a lower temperature than the flue gasinlet temperature.

While specific embodiments of the present invention have been shown anddescribed in detail to illustrate the application and principles of theinvention, it will be understood that it is not intended that thepresent invention be limited thereto and that the invention may beembodied otherwise without departing from such principles. In someembodiments of the invention, certain features of the invention maysometimes be used to advantage without a corresponding use of the otherfeatures. Accordingly, all such changes and embodiments properly fallwithin the scope of the following claims.

What is claimed is:
 1. A scale prevention system for a wet flue gasdesulfurization unit, the system comprising: a flue structure, whereinthe flue structure is connected to a flue inlet of a wet flue gasdesulfurization unit; at least two air nozzles, each air nozzle having arespective air supply line, wherein the at least two air nozzles arepositioned on the interior of the flue structure so as to provide aircoverage across the horizontal width of a flue inlet to the wet flue gasdesulfurization unit; and at least one inlet awning designed to deflecta slurry from the wet flue gas desulfurization unit, the at least oneinlet awning being position above the flue inlet to the wet flue gasdesulfurization unit.
 2. The system of claim 1, wherein the fluestructure has a rectangular or square cross-sectional shape and the atleast two air nozzles are positioned on opposite interior walls of theflue.
 3. The system of claim 2, wherein the at least two air nozzles arepositioned on opposite vertical interior walls of the flue.
 4. Thesystem of claim 1, wherein the at least two air nozzles receivetemperature controlled air that is within a range of about −25° F. toabout +25° F. of the wet flue gas desulfurization unit slurrytemperature.
 5. A scale prevention system for a wet flue gasdesulfurization unit, the system comprising: a flue structure, whereinthe flue structure is connected to a flue inlet of a wet flue gasdesulfurization unit and the flue structure has a cross-sectional shapeselected from a rectangle or a square; at least two air nozzles, eachair nozzle having a respective air supply line, wherein at least one airnozzle is positioned on each opposite vertical interior wall of the fluestructure so as to provide air coverage across the horizontal width of aflue inlet to the wet flue gas desulfurization unit; and at least twolateral air nozzles, each lateral air nozzle having a respective airsupply line, wherein each lateral air nozzle is positioned internally ofeach of the at least two air nozzles and are positioned at the top edgeof the flue inlet to the wet flue gas desulfurization unit so as toprovide air coverage across the horizontal width of the flue inlet tothe wet flue gas desulfurization unit.
 6. The system of claim 5, whereinthe at least two lateral air nozzles are separated by a baffle plate. 7.The system of claim 5, wherein the system further comprises at least oneinlet awning designed to deflect a slurry from the wet flue gasdesulfurization unit, the at least one inlet awning being position abovethe flue inlet to the wet flue gas desulfurization unit.
 8. The systemof claim 5 wherein the at least two air nozzles receive temperaturecontrolled air that is within a range of about −25° F. to about +25° F.of the wet flue gas desulfurization unit slurry temperature.
 9. A scaleprevention system for a wet flue gas desulfurization unit, the systemcomprising: a flue structure, wherein the flue structure is connected toa flue inlet of a wet flue gas desulfurization unit; and at least twochill plates, each chill plate having a respective supply line, whereinthe at least two chill plates are positioned on the interior of the fluestructure so as to provide temperature control across the horizontalwidth of a flue inlet to the wet flue gas desulfurization unit.
 10. Thesystem of claim 9, wherein each chill plate receives temperaturecontrolled gas.
 11. The system of claim 9, wherein each chill platereceives temperature controlled liquid.
 12. The system of claim 9,wherein the at least two chill plates receive a temperature controlledgas that is within a range of about −25° F. to about +25° F. of the wetflue gas desulfurization unit slurry temperature.
 13. The system ofclaim 9, wherein the at least two chill plates receive a temperaturecontrolled liquid that is within a range of about −25° F. to about +25°F. of the wet flue gas desulfurization unit slurry temperature.
 14. Thesystem of claim 9, wherein the system further comprises at least oneinlet awning designed to deflect a slurry from the wet flue gasdesulfurization unit, the at least one inlet awning being position abovethe flue inlet to the wet flue gas desulfurization unit.
 15. The systemof claim 9, wherein the flue structure has a rectangular or squarecross-sectional shape and at least one chill plate is positioned on eachof the opposite vertical interior walls of the flue structure.
 16. Thesystem of claim 9, wherein the flue structure has a cross-sectionalshape selected from a rectangle or a square and at least one chill plateis positioned on each of the opposite bottom interior corners of theflue structure.
 17. A scale prevention system for a wet flue gasdesulfurization unit, the system comprising: a flue structure, whereinthe flue structure is connected to a flue inlet of a wet flue gasdesulfurization unit; at least two air nozzles, each air nozzle having arespective air supply line; and at least two chill plates, each chillplate having a respective supply line, wherein the combination of the atleast two air nozzles and the at least two chill plates are positionedon the interior of the flue structure so as to provide temperaturecontrol across the horizontal width of a flue inlet to the wet flue gasdesulfurization unit.
 18. The system of claim 17, wherein the systemfurther comprises at least one inlet awning designed to deflect a slurryfrom the wet flue gas desulfurization unit, the at least one inletawning being position above the flue inlet to the wet flue gasdesulfurization unit.
 19. The system of claim 17, wherein the fluestructure has a rectangular or square cross-sectional shape and the atleast two air nozzles are positioned on opposite vertical interior wallsof the flue.
 20. The system of claim 17, wherein the flue structure hasa rectangular or square cross-sectional shape and at least one chillplate is positioned on each of the opposite vertical interior walls ofthe flue structure.
 21. The system of claim 17, wherein the fluestructure has a cross-sectional shape selected from a rectangle or asquare and at least one chill plate is positioned on each of theopposite bottom interior corners of the flue structure.
 22. The systemof claim 17, wherein the at least two air nozzles receive temperaturecontrolled air that is within a range of about −25° F. to about +25° F.of the wet flue gas desulfurization unit slurry temperature.
 23. Thesystem of claim 17, wherein the at least two chill plates receive atemperature controlled gas that is within a range of about −25° F. toabout +25° F. of the wet flue gas desulfurization unit slurrytemperature.
 24. The system of claim 17, wherein the at least two chillplates receive a temperature controlled liquid that is within a range ofabout −25° F. to about +25° F. of the wet flue gas desulfurization unitslurry temperature.
 25. A scale prevention system for a wet flue gasdesulfurization unit, the system comprising: a flue structure, whereinthe flue structure is connected to a flue inlet of a wet flue gasdesulfurization unit; at least two air nozzles, each air nozzle having arespective air supply line, wherein at least one air nozzle ispositioned on each opposite vertical interior wall of the flue; at leasttwo lateral air nozzles, each lateral air nozzle having a respective airsupply line, wherein each lateral air nozzle is positioned internally ofeach of the at least two air nozzles and are positioned at the top edgeof the flue inlet to the wet flue gas desulfurization unit; and at leasttwo chill plates, each chill plate having a respective supply line,wherein the at least two chill plates are positioned on the interior ofthe flue, wherein the combination of the at least two air nozzles, theat least two lateral air nozzles and the at least two chill plates arepositioned on the internal surfaces of the flue structure so as toprovide temperature control across the horizontal width of a flue inletto the wet flue gas desulfurization unit.
 26. The system of claim 25,wherein the at least two lateral air nozzles are separated by a baffleplate.
 27. The system of claim 25, wherein the system further comprisesat least one inlet awning designed to deflect a slurry from the wet fluegas desulfurization unit, the at least one inlet awning being positionabove the flue inlet to the wet flue gas desulfurization unit.
 28. Thesystem of claim 25, wherein the flue structure has a rectangular orsquare cross-sectional shape and the at least two air nozzles arepositioned on opposite vertical interior walls of the flue.
 29. Thesystem of claim 25, wherein the at least two lateral air nozzles areseparated by a baffle plate.
 30. The system of claim 25, wherein theflue structure has a rectangular or square cross-sectional shape and atleast one chill plate is positioned on each of the opposite verticalinterior walls of the flue structure.
 31. The system of claim 25,wherein the flue structure has a cross-sectional shape selected from arectangle or a square and at least one chill plate is positioned on eachof the opposite bottom interior corners of the flue structure.
 32. Thesystem of claim 25, wherein the at least two air nozzles receivetemperature controlled air that is within a range of about −25° F. toabout +25° F. of the wet flue gas desulfurization unit slurrytemperature.
 33. The system of claim 25, wherein the at least two chillplates receive a temperature controlled gas that is within a range ofabout −25° F. to about +25° F. of the wet flue gas desulfurization unitslurry temperature.
 34. The system of claim 25, wherein the at least twochill plates receive a temperature controlled liquid that is within arange of about −25° F. to about +25° F. of the wet flue gasdesulfurization unit slurry temperature.